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1. Field of the Invention
An embodiment of the present invention relates to a biological process for continues purification of wastewater by converting its constituents to a solid form that can be easily separated using retainable biological catalysts. An embodiment of the present invention also relates to a novel reactor hereafter referred to as xe2x80x9cReverse Fluidized Loop Reactorxe2x80x9d (RFLR) for performing the above said process.
2. Related Art Description
Microorganisms have been used for a long time for treatment of water as well as waste water. Some of the newer applications of micro-organisms include oxidation of sulphide and dissolved iron salts to elemental sulphur and iron slats of oxidized forms that can be are removed by precipitation respectively [see for example: Buisman, C. J. N., et al., Biotechnol. and Bioengng. 35, 50-56, (1990)]. Such applications are feasible in the case of dissolved manganese also. The biological removal of sulphide as mainly elemental sulphur, which is essentially insoluble in water, finds application in treatment of sulphide containing wastewater from a variety of industries, in particular, pulp and paper mill effluent and refinery and petrochemical effluents. Sulphide is also generated during the anaerobic treatment of wastewater containing sulfates such as distillery effluents and pharmaceutical effluents. Removal of hydrogen sulphide from gases by scrubbing with water or alkaline or carbonate absorbents also result in a sulphide containing liquor that can be treated and recovered by biological sulphide oxidation processes.
Dissolved iron is present in coal mining and other mining area drainages. These waters are highly acidic and require treatment. Biological oxidation converts dissolved ferrous iron to hydroxide or carbonate ferric ion precipitates which can be easily removed from water. Such a process has been applied for the treatment of acid mine discharges [see for example: Nakamura, K. et al., Water Research 20,1, 73-77 (1986)]. Another application for biological iron removal is for the treatment of drinking water particularly groundwater, which in certain regions contains unacceptable levels of dissolved iron. Manganese is also present in groundwater from certain regions, industrial effluents from steel and manganese plants and in drainage water from coal and iron ore mines. Removal of Manganese is also feasible by biologically oxidizing manganese to form insoluble manganese dioxide and hydroxide precipitates. Although, such processes are currently used only in the form of natural oxidation in systems such as constructed wet lands, it is conceivable that reactor systems operating at higher rate for the controlled removal of manganese by biological oxidation can be developed. Note that all these systems result in the formation of insoluble precipitates.
Another application which results in solid precipitate is biological sulfate reduction when applied for the removal of metals from wastewater. The reduction of sulfate to sulphide is carried out by organisms known as xe2x80x9csulfate reducing bacteria.xe2x80x9d These bacteria require an energy source or electron donor, which can be simple organic compounds such as methanol or gases containing hydrogen such as producer gas. The use of gas as the energy source is considered a more economic option for larger scale applications. Here again, we have a situation where the desired bacteriaxe2x80x94sulfate reducing bacteriaxe2x80x94is to be retained within the reactor, provided with a sparingly soluble gaseous reactant, and the final solid product is to be efficiently removed from the reactor.
Conventionally, the aerated reactors have utilized two techniques for maintaining a high concentration of micro-organisms in the reactor. In the widely used activated sludge reactors (see for example Metcalf and Eddy Inc. xe2x80x9cWastewater Engineering: Treatment, Disposal and Reusexe2x80x9d, Tata McGraw-Hill Publishing Co., New Delhi), sludge is recycled after separation by sedimentation from the effluent liquor. The high concentration of micro-organisms in the reactor takes the form of solid flocs kept in suspension by agitation or aeration in the reactor. The mixed and turbulent nature of suspension ensures effective contact of the biocatalystxe2x80x94i.e., the micro-organisms with the reactants i.e., oxygen and pollutant materials. When these systems are applied to processes which produce solid waste products, these products cannot be separated from the active biocatalysts in an ordinary sedimentation separation. Thus, there would be an accumulation of the products in the reactor, leading to lower efficiency and possible failure. It is conceivable that expensive post treatment measures to selectively remove the solid waste products from active biocatalysts would enable the functioning of the system.
It is also common to retain micro-organisms as a film on stationary inert packing materials inside the reactor. Such a device is called xe2x80x9cbiofilm reactorxe2x80x9d. These systems are better suited to anaerobic process which have no requirement of oxygen or aeration and have an intrinsic slow reaction rate as compared with aerobic systems. When applied to aerated process, the slow mass transfer of reactants to the stationary biofilm does not help in improving the performance.
In aerated process, the biofilm system takes the form of trickling filters where the liquid is sprinkled on top of reactor filled with packing materials which may be either natural random packing of stones or synthetic manufactured media in the form of random packing or structured packing. In these system, it is noteworthy that the liquid is present as discontinuous phase and air as continuous phase. It is also evident that such reactors are unsuited when solid products are generated during the reaction as these products will accumulate on the media. In fact, these systems are not recommended even when sedimentable inert solid are present in the raw wastewater for the same reasons.
There are some systemsxe2x80x94xe2x80x9caerated filtersxe2x80x9dxe2x80x94in which a synthetic packing, submerged in the liquid pool, is aerated from below using diffused aeration apparatus. Such systems again have all the oxygen mass transfer deficiencies of biofilm systems but have the advantage of retained biofilm thus avoiding post sedimentation, and being less affected by variability in the wastewater characteristics such as shock loads or transient toxic loads. It is evident that effective removal of solid products is not possible in the submerged aerated filter.
Another biofilm system is the fluidized bed bioreactor. Here the biofilm is present on carrier material which is retained in suspension within the bioreactor using the liquid velocity applied in upward direction. The constant state of agitation of the carrier particles ensures that mass transfer limitations of stationary biofilm reactors are minimized. The velocity applied is in the xe2x80x9cfluidizationxe2x80x9d regime, i.e.., the upward drag force applied on the biocarrier particle is equal and opposite to the buoyant weight of the particle. In solid-liquid fluidized bed reactors, there is a narrow range of velocity where this is effective. The situation is complicated by the application of aeration. At the top of the fluidized bed reactor, there is a disengagement section where gas, liquid and solids carried over are separated. The liquid velocity that has to applied is quite large and hence, energy consumption of fluidized bed reactors is usually higher than for other types of reactors. The fluidized bed reactor has been used for anaerobic wastewater treatment applications but for aerated systems such application is rare due to the hydrodynamic complication of maintaining stability in a 3 phase fluidized bed. The removal of solid products is also problematic as it requires application of velocities that would carry out the solid product while ensuring the retention of biocarrier particles. It would further reduce the stability of the operating regime.
The concept of the airlift fermentor uses a draft tube system with aeration to set up a circulation flow. This system has been developed and used for conduct of biological reaction without the presence of biocarriers. An extension of the air lift reactor called the biofilm air lift reactor, using biocarriers, has been developed for wastewater treatment [Heijnen J. J. et al., Chem. Eng.Technol., 13,202-208 (1990).]. It has been commercially realized in several installations. The biocarriers are in fluidized state or in circulation in these reactors. The biofilm air lift reactors have a wider range of hydrodynamic operability as compared with the 3 phase fluidized bed reactor.
The concept of reverse fluidized bed reactor refers to the use of biocarrier particles that have a specific gravity lower than the fluid (usually wastewater). The bed of biocarrier particles forms a floating bed on top of the reactor. Fluidization is achieved by the application of fluid velocity in the downward direction. The basic advantage of such a system is the ability to remove solid products by the combined action of sedimentation and concurrent flow of liquid. However, this system is very difficult to realize with aeration because of the instability and very narrow range of downward fluidization velocities. Further, the uniform distribution of liquid to the top of the reactor to enable fluidization without channeling and the removal of rising gases at the same time presents technical problems, so much so that the reverse fluidized bed reactor has not been realized in any practical application. Aeration is only practically possible in an external loop to the reactor and which sets additional limits to the capacity of the reactor.
Reference may be made to Garcia-Calderon, D. et al., Water Res., 32(12), 3593-3600, 1998 and Garcia-Bernet, D. et al., Water Sci. Technol., 38(8-9), 393-399, 1998 wherein the downflow fluidized bed reactor or inverse fluidized bed reactor has been described for application in anaerobic treatment of wastewater. In their description, of down-flow fluidization, particles with a specific gravity smaller than the liquid are fluidized downward by a concurrent flow of liquid. The paper describes the application of the downflow (or inverse) fluidization technology for the anaerobic digestion of red wine distillery wastewater. The carrier employed was ground perlite, an expanded volcanic rock. The biofilm formation and its effect on hydrodynamics of the reverse fluidized bed reactor has been described [Garcia-Calderon, D. et al., Biotechnol. Bioeng., 57(2), 136-144 , 1998]. It should be noted that all these work has been done for anaerobic system not to aerated systems.
The application of inverse fluidization in wastewater treatment from laboratory to full-scale bioreactors has been described in a paper by Karamanev, D. G. and Nikolov, L. N.; Environ. Prog., 15(3), 194-196, 1996. Here, the inverse fluidized bed biofilm reactor is designed so that the biofilm thickness can be controlled to avoid the intrabiofilm diffusional limitations. The basis of the reactor is a draft-tube airlift apparatus. The circulating liquid expands the bed of buoyant particles in the annulus. Initially, the lower bed boundary is well above the lower tube opening. The biofilm, growing on the surface of support particles, increases the overall bioparticle (support particle plus biofilm) diameter. It results in bed expansion and very slow movement of the lower bed level downward until the lower bed level reaches the lower draft tube opening and some of the bioparticles enter the draft tube with the liquid flow. There, due to the strong shear stress, part of the biofilm is removed and the biofilm thickness decreases. Eventually, these bioparticles exit the draft tube and enter the top of the annulus, where the process repeats. This controls the biofilm thickness. In this description, the inverse fluidized bed operates primarily as an expanded fluidized bed, the loop circulation of biocarriers mainly for removal of excess biofilm from heavier particles. There is no intent to use the reactor system for production and removal of solid products in biological sulphide oxidation or iron oxidation or manganese oxidation.
An apparatus for biological treatment of wastewater in downflow mode is disclosed by Shimodaira; U.S. Pat. No. 4,454,038, filed on Oct. 31, 1980, issued on Jun. 12, 1984. A previous patent of same inventors, U.S. Pat. No. 4,256,573, discloses a processes of biological treatment of wastewater, wherein a downflow fluidized bed is utilized for anaerobic and aerobic wastewater treatment. The basic apparatus disclosed is a downflow fluidized bed with lighter than water biocarrier provided with distributor for uniform liquid distribution on top of the reactor vessel. The apparatus is described for use in nitrification, denitrification and BOD removal. An improvement on the basic apparatus is claimed wherein a draft tube is provided with an aeration system, whereby aeration set up circulation for providing fluidization. The function of the draft tube claimed is for providing an internal loop flow of the liquid for aeration rather external loop flow of liquid by pumping. There are significant and crucial differences between an embodiment of the present invention and those disclosed by Shimodaira. These are enumerated below:
The previous apparatus is not designed or intended for removal of solids. An embodiment of the present invention is specifically for the production and removal of solids, thereby effecting treatment. Thus an embodiment of the present invention has special utility in the biological removal of sulphide, iron and manganese from wastewater.
The apparatus disclosed in the previous invention is a genuine fluidized bed i.e., the intended operation is bed expansion of substantially constant length while an embodiment of the present invention is for a fluidized circulating bed. The maintenance of constant bed expansion is said to be aided by injection or gas formation in the bed. It is crucial to have bed circulation for an apparatus that is intended for reactions that result in the production of solids, in order to achieve adequate removal of inert materials.
The apparatus disclosed in the previous invention envisages carrier material only in the annular space between the draft tube and reactor wall, and hence no significant portion of the reaction inside the draft tube. An embodiment of the present invention on the other hand, has substantial portion of the carrier inside the draft tube in upward motion and significant portion of the reaction takes place inside the draft tube. It is also remarkable that an embodiment of the present invention in a preferred arrangement uses draft tube of diameter such that the cross sectional area of the draft tube is larger than the cross sectional area of the annular space between the draft tube and reactor wall.
The apparatus described in the previous invention requires specially designed liquid distribution to uniformly distribute liquid at the surface of the bed and several arrangements for this is described. Whereas, an embodiment of the present invention being a circulatory fluidized bed, requires no special liquid distribution mechanism. Circulation velocity applied is qualitatively greater than fluidization velocity.
An embodiment of the present invention and its operating regime is specially suited for carrying out biological reactions that produce solid products like elemental sulphur, iron oxides and manganese oxides, which are inorganic, generally of specific gravity substantially greater than biomass and are in finely divided form.
In U.S. Pat. No. 5,019,268, an apparatus is described for aerobic biological wastewater treatment, that comprises lighter than water fluidized beds essential in upflow configuration, but which has to be periodically flushed by downward fluidization for removal of captured solids which may be present in the wastewater. An embodiment of the present invention has a completely different arrangement and purpose.
Oh, Kwang-Joong; et al., Korean J. Chem. Eng., 15(2), 177-181 (1998) Korean Institute of Chemical Engineers, describes a process for removal of hydrogen sulphide by biological oxidation in a three phase fluidized bed bioreactor. Thiobacillus sp. was immobilized on biosands.
The problems with using carrier particles and fixed beds for sulphide oxidation are discussed in a patent (C. J. N. Buisman, U.S. Pat. No. 5,637,220) wherein it is disclosed that product sulphur itself retained in the reactor may serve as biocarriers. This arrangement with large inventory of sulphur held within the reactor is subject to instability due to intense and fast backward reaction to reform sulphide from sulphur when there is failure of aeration or during shutdown. An embodiment of the present invention overcomes these disadvantages by ensuring almost negligible hold-up of sulphur in the reactor. An embodiment of the present invention maintains superior activity of the biocatalysts compared with the disclosed invention because of the lack of contamination of biomass with sulphur. A further advantage of an embodiment of the present invention is the purity of product sulphur is superior as it is not contaminated with biological agents which are retained substantially on attached biofilm.
The use of synthetic material such as plastics like PVC rather than ceramics or sand or activated carbon in three-phase fluidized bed bioreactor is not common. It is noted that here again, the carrier material has specific gravity higher than the fluid. Micro-organism carriers of specific shape and of specific gravity less than that of water for fluidized bed application and their production is described in patents e.g. [JP 11000682 A2 6, January 1999]. Micro-organism carriers made of plastic with variable specific gravity and combination of fluidized bed reactor with a flotation separation system has been described in patents [JP 10192878 A2 28, July 1998]. However, it is noted that the considerable advantage of carriers so as to achieve reverse fluidization in loop flow configuration system is not previously claimed. Further, the essence of an embodiment of the present invention is that it can be applied for the conduct of biological reactions that result in solid products and is effective in removal of solid wastes while retaining active biocatalysts in the reactor.
The use of a sand fluidized bed for oxidation and removal of iron is claimed in a patent application [PCT Int. Appl. WO 9406717 A1 31, March 1994]. The difficult step of separation of oxide precipitates is achieved using high velocity and thus at high energy cost. In an embodiment of the present invention, the separation is efficiently achieved at lower cost by the use of carrier particles lighter than water.
It is therefore strongly desirable to have a reactor which achieves high rates of reaction for the above processes requires to retain the desired population of micro-organisms within the reactor, a sufficiently high number of slow-growing biological agents will be available inside the reactor at all times to carry out the desired reactions and the purity of the effluent and products will be enhanced without passing through a separation stage for the removal of organisms. The reactor should have better capacity to withstand shock loads or starvation phases that tend to inactivate the organisms.
Embodiments of the present invention may be seen as a synergistic combination of airlift biofilm with a downflow fluidized bed, which results in an unexpected, surprising and effective result, particularly when used in a biological wastewater treatment processes wherein solid particles are obtained as a result of the reaction. As such, an embodiment of the present invention comprises a process for the purification of wastewater by the conduct of biological reactions involving the production of solids using biological agents and at least one gaseous reactant. Another embodiment of the present invention comprises a process that can efficiently separate solid biological agents from other solids present in the liquid or generated during the conduct of the reaction for the purification of liquid. Still another embodiment of the present invention comprises a process that enables continuous input of reactants and continuous discharge of liquid products while retaining biological catalysts for continuous reuse. Yet another embodiment of the present invention comprises a process that enables very efficient continuous discharge of solid products of the purification reaction while retaining active biological catalysts for continuous reuse. One more embodiment of the present invention comprises a process that enables mixing of gaseous and liquid reactants and biological catalysts so as to create conditions for the effective conduct of the reaction. One other embodiment of the present invention comprises a process where the activity of biological catalysts can be maintained and prevented from contamination by solid products whereby reduction of reaction rate as a result of mass transfer of reactants is avoided.
Accordingly, embodiments of the present invention relate to a biological process for purification of wastewater using retained biological catalysts. Embodiments of the present invention also relate to a novel reactor hereafter referred to as a xe2x80x9cReverse Fluidized Loop Reactorxe2x80x9d (RFLR) useful in performing the above said process.